The invention relates to a method and a device for treating bodies of water, preferably flowing water in irrigation canals, with acrolein as a biocide. Acrolein is added in order to avoid algal growth and excessive growth of weeds in these bodies of water. In the method according to the invention, an acrolein acetal is used as a biocide precursor. The acrolein acetal is deacetalated in the device according to the invention. No electrical energy is required to operate the device.
In warm climatic zones where there are extensive fields and plantations under agricultural use, a large network of irrigation canals is maintained. In these canals, algal growth and excessive growth of weeds easily occur because of the presence of aquatic plants. The plants hamper the flow rate in the canals and impede the trouble-free action of the pumping stations necessary for operation. For this reason, it is customary to treat the water in such irrigation systems with a biocide.
Various biocides may be used for doping these stretches of flowing water. In practice, acrolein has proven to be particularly successful as a biocide.
Apart from its hitherto unsurpassed activity, acrolein also has an advantage over other biocides in that it undergoes degradation in water after a relatively short time. Acrolein is thereby no longer active as a biocide at the point of irrigation of the fields, but it is degraded into decomposition products that do not harm the physiology of plants.
Up to the present, the doping of water with acrolein has involved applying excess pressure on a storage tank filled with acrolein by means of a pressurized gas from a pressure cylinder and moving the acrolein through an ascending pipe from the storage tank directly into the irrigation canal. However, handling is risky because of the physical and chemical properties of acrolein. Acrolein is poisonous and highly toxic if inhaled, as well as pungent and lachrymatory. The product is easily flammable (flash point -29.degree. C., boiling point 53.degree. C.), which creates handling problems in warm climatic zones. Furthermore, there is a danger of explosive polymerization reactions if the acrolein becomes contaminated with impurities.
In view of the risks described, which arise both during the transport and in the use of acrolein, there is a need to carry out doping with a biocide having comparable activity, but wherein the handling of the biocide is considerably less risky.
The hydrolysis of acrolein acetals, also including cyclic acrolein acetals, with the recovery of acrolein, is known from U.S. Pat. No. 4,851,583 (which patent is entirely incorporated herein by reference). Strongly acidic ion exchangers are used as catalysts. But the method described in this patent is unsuitable for doping stretches of flowing water, because it rapidly leads to the contamination or inactivation of the ion exchanger. Moreover, most acrolein acetals have only a very limited solubility in water, and, in addition, the rate of solution is low. For example, to prepare a solution of 2-vinyl-1,3-dioxolane (VDL) in water, which under practical conditions is almost saturated (approximately 8% by weight), it is necessary to stir for approximately 10 minutes with intimate mixing. At a water volumetric flow rate of 30,000 m.sup.3 /h and strong algal growth, for the purpose of adequate doping, 300 liters of VDL/h to 3750 liters water/h would have to be homogenized and then deacetalated. These factors have hitherto hampered the use of acrolein acetals for liberating acrolein in the large quantities which are necessary for the treatment of irrigation canals, in particular those having a high content of algae and aquatic plants, because under practical conditions, no electrical energy is available for the purpose of intimate mixing.
One method of doping water is described in German Patent Publication No. P 40 38 471 (which is also entirely incorporated herein by reference). According to the teaching of this publication, acrolein is formed by deacetalation of acrolein acetals in the aqueous phase in the presence of a strongly acidic catalyst. In this connection, an acrolein di-n-alkylacetal, wherein the alkyl groups contain 3 to 5 carbon atoms, or a cyclic acrolein acetal, wherein the alcohol components have 2 to 6 carbon atoms and 2 to 6, preferably 2 to 4, OH groups, is used. A strongly acidic ion exchanger or a mineral acid is used as a deacetalation catalyst. The method for doping aqueous solutions described in German No. P 40 38 471 is based on the principle that the acrolein formed during deacetalation in a reaction vessel is continuously removed from the aqueous phase and transferred into the aqueous solution which is to be doped. This transfer takes place either by means of a stream of gas which is passed through the reaction vessel and then conducted into the aqueous solution to be doped, or by using a liquid jet pump, wherein the pumping fluid is the aqueous solution to be doped. This method is in fact very suitable, for example, for doping cooling-water systems in industrial plants, but in practice, it cannot be used for doping stretches of flowing water such as water in irrigation canals, because the electrical energy which is necessary for operating the pumps is generally not available at the site where doping takes place. Doping stretches of flowing water with a biocide must be feasible in the "open field", where neither electrical energy nor chemically-trained technical staff are available to operate the appropriate devices.